gluPerspective specifies a viewing frustum into the world coordinate system.
In general, the aspect ratio in gluPerspective should match the aspect ratio
of the associated viewport. For example, aspect=2.0 means
the viewers
angle of view is twice as wide in x as it is in y.
If the viewport is
twice as wide as it is tall, it displays the image without distortion.

The matrix generated by gluPerspective is multipled by the current matrix,
just as if glMultMatrix were called with the generated matrix.
To load the perspective matrix onto the current matrix stack instead,
precede the call to gluPerspective with a call to glLoadIdentity.

Depth buffer precision is affected by the values specified for
zNear and zFar.
The greater the ratio of zFar to zNear is,
the less effective the depth buffer will be at distinguishing between
surfaces that are near each other.
If

r = zFar / zNear

roughly log 2r bits of depth buffer precision are lost.
Because r approaches infinity as zNear approaches 0,
zNear must never be set to 0.